1. Field of the Invention
This invention pertains to the art of induction heating and more particularly to a method and apparatus to uniformally galvanneal edges of coated strip steel.
2. Description of Related Art
Galvanizing is the coating of iron or steel products with zinc to inhibit rust. Galvannealing is the process of further treating a zinc-coated steel strip for forming an extended iron-zinc alloy. A galvannealed product has a number of advantages over a galvanized product, e.g. easier spot welding and better paintability.
The galvannealing process essentially consists of submerging a clean, preheated steel strip in a bath of liquid zinc or zinc-rich alloy. As the strip emerges from the bath, it passes through a coating control system, such as an air knife, which is used to control the thickness of the zinc coating. The coated steel strip is then reheated in a galvannealing furance to produce further intermetallic diffusion between the zinc and steel substrate. A galvannealing furnace is typically fuel fired, but it is also known that the heating may be done electrically by induction heating coils.
There are at least two types of galvannealed products: one has equal coating on either side of the steel strip, the other is described as an "AB product" and has a different coating thickness or "weight" on each surface. The different coating weight is controlled by adjustment of the air knife. In an AB product, essentially all the free zinc is removed from one surface while the other surface has various coating weights remaining.
A particular problem which has been encountered with galvannealing in installations that employ induction heating coils is the occurrence of lines or stripes in the coated steel product. The stripes appearing in the finished product are typically parallel to the direction of movement of the strip through the furnace. Occurrence of the stripes is coincident with an audible noise so that the characteristic has been referred to as "noise stripes." The stripes are actually evidence of a resonant response in the strip to the induction heating field of the induction heating coils. U.S. Pat. No. 4,807,559 to Sommer, et al. discloses an induction heating furnace wherein the operating frequency of the heating coils is modulated to avoid a resonant vibration in the steel strip.
Because steel strip will often be cooled faster at the edges than in the middle, non-uniformities in the galvannealing process are introduced. Such non-uniformities are thought to be introduced by the air knives. This heat transfer phenomenon of the edges of a strip being cooled faster than the middle of the strip introdues non-uniformities in the strip. Designers have sought to minimize or eliminate this problem through a variety of mechanisms. In U.S. Pat. No. 4,627,259 to Anderson, et al., an edge region heating device is disclosed for use in the hot rolling of a metallic strip after some cooling of the strip has occurred. The device employs an induction heater to preferentially raise the temperature of the edge regions of the strip to compensate for the faster cooling of these regions. Similarly, Japanese Patent No. 7209253 discloses a strip rolling apparatus in which the edges of the strip are maintained at the correct temperature by passing through induction coils.
The above-referenced edge region heating devices may suffer from noise stripes just as a larger galvannealing induction furnace normally would. The present invention contemplates a new and improved method and apparatus to overcome the above-referred to problem and others.